The machines used hitherto for implementing this lost-rod method are, in principle, conventional drilling machines. These machines are designed for drilling the tap hole with the aid of a drill bit coupled to a work member. This work member, which can be moved along the mount through the action of a drive means, consequently comprises a rotary drilling machine, a hammer supporting the drilling operation and a chuck for receiving the drill bit. These conventional drilling machines have undergone a number of transformations so that they can also be used for the above-mentioned application of the lost-rod process. Thus, the work members have been equipped with a powerful two-directional hammer for developing the energy necessary for the insertion and the removal of the boring rod, and the chuck has been replaced by a means for transmitting, to the rod, the energy thus developed by the hammer, for example with a clamp for gripping the free end of the rod.
In point of fact, a powerful hammer, as used on these machines, is not without drawbacks. Firstly, it imposes considerable stresses and vibrations on the equipment, particularly on the clamp for coupling with the rod which is therefore subject to rapid wear. It is also extremely noisy and often does not meet the increasingly severe standards aimed at reducing the level of noise in the industrial environment.
Patent Application EP 0,379,018 (corresponding to U.S. patent application No. 462,415 assigned to Paul Wurth S.A., the disclosure of both of which are incorporated herein by reference) provides indications for partially remedying these drawbacks. It proposes a multi-stage process for removal of the rod, using a to-and-fro movement of a clamp through the action of a silent hydraulic jack. The pneumatic hammer, which must no longer be two-directional since it is no longer used for removal, is nevertheless still necessary for efficient and rapid positioning of the boring rod.
The drawbacks of the hammer also encourage elimination of the hammering operation during the insertion stage. For example, it would be possible to envisage subjecting the work member to a more powerful drive means in order to cause the rod to penetrate forcibly and without vibrations into the semi-hardened sealing mass. Unfortunately, this plan seems a priori to be unworkable. In fact, given the length of the rod, a more powerful thrust without a hammer increases the risk of, firstly, buckling of the rod and, then, its permanent immobilization in a partially driven-in position in the mass which is rapidly hardening.
An ingenious solution to the problem of forceable insertion of the rod is suggested in Luxembourg Patent Specification LU-87 915 (corresponding to U.S. patent application No. 862,487 assigned to Paul Wurth S.A., the disclosure of both of which are incorporated herein by reference). This patent proposes a two-directional clamp whose alternating to-and-fro movement at the front of the mount is used both for insertion and for removal of the boring rod. This proposal thus seems to be the optimum solution in that it makes it possible to dispense completely with the noisy and destructive hammer of the clamp. A disadvantage of this machine is that the two-directional clamp and the means which actuate it are located in the front region of the mount. In point of fact, this region is a zone which is at risk from splashes when the molten stream issues from the tap hole after removal of the rod.